Secondary electron amplifier



Oct. 1. 194 E. SCHWARTZ ET AL $EGONDARY ELECTRON AMPLIFIER 2 Sheets-Shes t 1 Filed Feb. 9. 1937 ffw'ch Sewn/6T2, fled/Math -5272129151,.

Patented Oct. 1, 1940 PATENT OFFlCE SECONDARYELECTRON AMPLIFIER Erich Schwartz, Berlin-Zehlendorf, and Heinrich Striibig, Teltow, near Berlin, Germany, assignors to the firm Fernseh Aktien-Gesellschaft,

. Zehlendorf, near Berlin, Germany Application February 9, 1937, Serial Nd 124,936

, In Germany February 10, 1936 4 Claims.

The invention relates to amplifier tubes in which the emission'of secondary electrons from anode surfaces is used for amplification, These tubes usually operate in the, mannerthat electrons of a certain velocity liberate secondaries from an electrode. These liberated electrons im pact a second secondary-emitting electrode, and these secondaries impact a third secondaryemitting electrode, and so forth.

In these tubes, in which the emitting'electrodes should possess surfaces capable of high secondary emission,-the amplification is limited by the in creasing current density because of space charge. Furthermore, the life of the amplifier tube is greatly decreased by too intense an electron bombardment of the secondary-emitting surfaces.

It is the object of the invention to arrange the electrodes of subsequent stages in such a manner 39: that the current density stays within reasonable limits even in the last stages, in spite of a high amplification factor. According to the invention, this is achieved by giving the emitting electrodes an area, which increases from stage to stage.' This arrangement also givesa decrease in space charge.

The whole amplifier by secondary emission may be built in a concentric arrangement. Electrodes then have the shape of plane circular rings or truncated cones of increasing diameters, instead of the usual electrodes which have the same area in all stages.

In order to decrease the space charge still more, auxiliary electrodes, for instance, grids or other permeable electrodes, are incorporated, and the distances between these auxiliary electrodes and the emitting electrodes is chosen in such a manner that in spite of a large field strength in front of the emitting electrode, the primary electrons 40 still impact the secondary emitting electrode.

The drawings show embodiments of the invention.

Figure 1 shows an amplifier with secondary emission according to the invention. 45 Figures 2 and 3 show another example of a multi-stage amplifier.

Figure 4 shows an amplifier tube with secondary emission with a magnetic field according to the invention.

Figure 5 shows another embodiment.

In Figure 1, vacuum receptacle l5 contains the thermionic cathode [6 for emitting primary electrons which are varied in number by the control grid IT. The primaries impact the cone-shaped 55 first secondary-emitting electrode l8, and liberate secondary electrons which leave the cone 3 substantially perpendicularly to the axis of the cone, and impact the second secondary-emitting electrode IS. The latter has the shape of a truncated cone, whereby its inside surface is impacted by electrons, so that further secondary electrons are liberated. These are accelerated in the direction of the third electrode 29, also having the shape of a truncated cone, and impact the outer wall of this electrode. The electrodes 2!, 22 and 23 are similar to the electrodes i9 and 2t, and fulfill the same functions. The cone l8 and the truncated cones and 22, and the truncated cones 19, 2| and 23 complement each other to form two concentric cones. The cloud of secondary electrons swings in the space between these two cones, and is finally collected by the collecting anode 24. It may be seen from Figure 1 that the area of the secondary-emitting surfaces increases from stage to stage, whereby space 2% charge and stress of the emitting electrodes is lower than in any previously known arrangements. As the space charge density increases, according'to a logarithmic law, it is advisable to let the area of the secondary-emitting electrodes 25 also increase, according to a logarithmic law, in order to obtain constant space charge. When using a cone shape, a circular surface for the emitting electrodes is obtained, the generating line of which is a logarithmic line.

The voltages applied to the emitting electrodes are chosen in such a manner that the gain of secondary electrons is as large as possible in every stage of the amplifier. The electrode I 8 is, for instance, given a positive voltage of 450 volts with respect to the cathode; the electrode I9, 900 volts; the electrode 20, 1350 volts; the electrode 2|, 1800 volts; and so on. The auxiliary electrodes I, 8, 9, l0 and M, if incorporated in the tube to insure drawing of the secondaries away from the emitting surface, are, for instance, given the potential of the next following emitting electrode and may be directly connected with the latter.

If the subtended angle of the cone is enlarged to 180, an arrangement is obtained in which the electrodes consist of concentric flat cylindrical rings. Such an arrangement is shown in Figures 2 and 3. The first emitting electrode l is given the shape of a round plate. Secondary-emitting electrodes 2, 3, 4, 5 and B have the shape of a fiat cylindrical ring, so that the areas of the electrodes increase proportionally to the radius with increasing number of stages. The collecting anode 24 is given the shape of a cylinder.

The invention is not limited to amplification by means of secondary emission with electrostatic fields. Devices in which the paths of the secondary electrons are controlled by means of magnetic deflection fields are also within the scope of the invention. In Figure 4, the cylindrical metal receptacle has welded or soldered to the middle of the bottom of it a vertical metal pin 26; a direct current source 21 is connected between the end of this metal pin 26 and the upper end of the metal receptacle 25, so that a magnetic field may be formed in the receptacle and its axis, the magnetic lines of force filling out the entire interior of the receptacle in the form of complete circles. It is naturally possible to substitute a coil of suitable shape for this metal receptacle, in order to decrease the direct current required in the arrangement. receptacle 25 may be either directly placed into a vacuum receptacle, or a vacuum receptacle 38 may be arranged in the interior of the metal vessel. In this vacuum vessel there are concentric fiat cylindrical plates 29, 30 and 3i as secondary emitting electrodes, an electron source, as for example a thermionic or photo-cathode 32, and the accelerating electrodes 33, 34 and 35. The combination of the magnetic field and the electrostatic field between the electrodes causes semi-circular paths of the primary electrons on which they travel to the following secondary emitting electrode. As it is not possible to obtain a homogeneous magnetic field inside of the metal receptacle 25, the electron paths will have a greater radius as they come nearer to the outside of the receptacle, and the width of the individual fiat circular rings will be accordingly increased, in order to insure that the primary electrons of one ring impact the following secondary-emitting electrode.

A further embodiment is shown in Figure 5. In this case, the electrodes have a semi-circular shape, and are placed in a semi-circular vacuum vessel 36. The magnetic field is generated by a U-shaped magnet 31 in the front of the poles of which the vacuum vessel is arranged. The magnetic field has an approximately semi-circular shape within the range of the vacuum vessel, so that the electrons may have arch-shaped paths The metal 1 from the innerplates to the outerplates, similar to the device shown in Figure 4.

Having thus described our invention, we claim:

1. An electron multiplier comprising a source of primary electrons, a series of secondary-emissive truncated cone-shaped electrodes of increasing mean diameter supported concentrically about a common axis with the inner surfaces of alternate truncated cone-shaped electrodes exposed to electron bombardment by electrons from the outer surfaces of electrodes intermediate said alternate electrodes, a secondary-emissive coneshaped electrode supported at a point on said axis and responsive to electrons from said primary source for bombarding the first of said truncated cone-shaped electrodes in the series with secondary electrons, and a collector electrode for collecting electrons from the last of said truncated cone-shaped electrode in said series.

2. An electron multiplier comprising an envelope having therein a plurality of secondaryemissive electrodes of truncated cone shape, each of said electrodes having a mean diameter greater than that of the preceding electrode in the series, the inner surfaces of alternate ones of said electrodes being exposed to bombardment by electrons from the outer surfaces of those of said electrodes intermediate said alternate electrodes.

3. An electron multiplier comprising an envelope having therein a plurality of secondaryemissive electrodes of truncated cone shape, each of said electrodes having an area greater than that of the preceding electrode in the series, the inner surfaces of alternate ones of said electrodes being exposed to bombardment by electrons from the outer surfaces of those of said electrodes intermediate said alternate electrodes.

4. An electron multiplier comprising an envelope having therein a plurality of secondaryemissive electrodes of annular shape, each of said electrodes having an area greater than that of the preceding electrode in the series, the inner surfaces of alternate ones of said electrodes being exposed to bombardment by electrons from the outer surfaces of those of said electrodes intermediate said alternate electrodes.

ERICH SCHWARTZ. HEINRICH STRZU'BIG. 

